Solid state isolation device using opto-isolators

ABSTRACT

An improved solid state isolation device that uses opto-isolators to provide an electrical barrier between a telephone network and user telephone equipment includes a load and current control circuit, and an adjustable termination circuit to eliminate or reduce signal distortion and insensitivity to low signal levels resulting from low loop current, and lack of complex impedance compensation/termination for diverse telephone system parameters. An improved trans-hybrid return loss circuit reduces internal losses; while loop termination losses are reduced by the use of Shottky diodes in the line coupler circuit. A high pass filter improves common mode rejection ratios in the transmit and receive paths, and adjustable compensation is provided to account for manufacturing variances in the opto-isolators. An additional connection is also provided to adapt the opto-isolator circuit for use with 3-wire ring circuitry.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to isolation circuitry. More particularly,the present invention relates to an improved solid state isolationdevice such as used in telephone line interface equipment.

2. Description of the Prior Art

Devices that isolate user telephone equipment from telephone networksare required by government regulatory agencies, such as the U.S. FederalCommunications Commission, to protect the telephone networks fromanomalies related to the user equipment, for example as might resultfrom the use of nonstandard or defective equipment or the inadvertentcoupling of line voltages into the telephone network. Isolation alsoprotects the user and user equipment from telephone network-relatedanomalies, such as voltage surges due to power supply fluctuation andlightning. A discussion of the state of the art with regard to isolationdevices for telephone networks is provided in D. Wilkison, D. Lee, SolidState Isolation Device Using Opto-Isolators, U.S. Pat. No. 5,245,654 (14Sep. 1993).

FIG. 1 is a block schematic diagram of an isolation circuit disclosed inU.S. Pat. No. 5,245,654. The figure shows a state of the art isolationcircuit, also referred to as a direct access arrangement ("DAA") 10,that interfaces a user device 12, such as a modem, to a telephonenetwork. The network is shown as a pair of signal lines 15T and 15R,often referred to as "tip" and "ring", respectively. The DAA 10 couplesa pair of analog channels--transmit and receive--to the telephonenetwork, while providing a high level of DC isolation between the userdevice and the lines 15T and 15R. The DAA also provides surgeprotection, rectification, off-hook detection, and ring detection.

The DAA 10 includes an isolation circuit 20 that is coupled to a userdevice 12 via transmit and receive lines 22, 23, and a line coupler 25that is connected to the lines 15T and 15R. The isolation circuit andthe line coupler communicate via a pair of signal lines 35, 37, that arealso designated as L+ and L-, and which generally correspond to tip andring.

The isolation circuit includes a transmit optical isolator circuit 50, areceive optical isolator circuit 52, and a hybrid 55. The opticalisolator circuit 50 is disposed between the user device transmit channeland the hybrid; and the optical isolator circuit 52 is disposed betweenthe receive channel and the hybrid. The optical isolator circuits 50, 52serve to communicate analog signals across an isolation barrier, shownin the figure as a dashed line 57, while preventing an electricalconnection across the barrier. The hybrid 55 interfaces thetwo-conductor line circuit, L+ and L-, to separate transmit and receivechannels, which typically consist of four conductors, to permit fullduplex operation.

The many advantages of using optical isolation instead of isolationtransformers are discussed in U.S. Pat. No. 5,245,654, which isincorporated herein by this reference thereto. It has been found inpractice that the use of optical isolation techniques to interface userequipment to a telephone network has presented unique problems andlimitations, especially when such approach is applied to telephonenetworks having disparate operating parameters and ranges, such as areencountered in the world's many different telephone standards andsystems.

Such problems and limitations include, for example:

1. Signal distortion and insensitivity to low signal levels resultingfrom low loop current, lack of complex impedancecompensation/termination for diverse telephone system parameters, and/orinternal circuit losses;

2. Limited physical interface capability, for example in applicationsrequiring 3-wire ring connectivity;

3. Poor common mode rejection ratios in the transmit and receive paths;and

4. Lack of compensation for manufacturing variances in circuitcomponents, especially variances in the opto-isolators.

It would be advantageous to improve known telephone network opticalisolation circuits, such that the various problems and limitationsattendant therewith are overcome.

SUMMARY OF THE INVENTION

The invention provides an improved solid state isolation device thatuses opto-isolaters to provide an electrical barrier between a telephonenetwork and user telephone equipment.

In particular, the invention provides a novel load and current controlcircuit and adjustable termination to meet different country standardsfor return loss and to improve sensitivity to low signal distortionlevels resulting from low loop current. The invention also addresseslack of complex impedance compensation/termination for diverse telephonesystem parameters. The load and control circuit includes a Zener diodecontrolled voltage regulator that uses the loop current to establish areference voltage for the receive path opto-isolator. A referencevoltage is also established for the transmit AC line driver circuit. Inthis way linearity is improved and distortion is reduced, especially forapplications involving low or varying loop current levels. The load andcontrol circuit, along with an adjustable termination circuit alsoprovides any desired load impedance to match the opto-islator circuit tothe telephone system with which it is used

Internal circuit losses are reduced through the use of an improvedtrans-hybrid return loss circuit; while loop voltage losses are reducedby the use of Shottky diodes in the line coupler circuit. Such lossreduction improves opto-isolator circuit performance in applicationsproviding low loop currents, such as those applications involving longcentral office loops.

A high pass filter is provided to improve common mode rejection ratiosin the transmit and receive paths, and adjustable compensation isprovided to the opto-isolator drivers to account for manufacturingvariances in circuit components, especially variances in theopto-isolators. An additional connection is also provided to adapt theopto-isolator circuit for use with 3-wire ring circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram of a prior art isolation circuit ofthe type that uses opto-isolators to interface a user device, such as amodem, to a telephone network;

FIGS. 2a-2c provide a schematic diagram of an improved solid stateisolation device using opto-isolators according to the invention; and

FIG. 3 is a graph plotting a load resistance window for an U.S.telephone standard and a German telephone standard; and

FIG. 4 is a graph plotting a load resistance window for a United Kingdomtelephone standard.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2a-2c provide a schematic diagram of an improved solid stateisolation circuit 20 using opto-isolators according to the invention.The invention is architecturally similar to that of the prior circuitshown in FIG. 1, but resides in various improvements to such circuit, asdiscussed below. Common features of both the prior art and the inventionshall be referred to herein by the same numeric designator.

The circuitry on the user device side of the isolation barrier 57requires electrical power from a source other than the phone line tooperate. Therefore, the isolation circuit is best suited for user devicethat can supply such power. For example, if the device is a modem, themodem side of the isolation circuit receives its power from the modem orcomputer supply. The line side of the isolation circuit receives itspower from the DC voltage, nominally 48 volts before attenuation, on theline when the off-hook relay is closed. The isolation circuit operatesto couple AC signals across the barrier 57, and so references tosignals, voltage, and currents herein generally means AC signals,voltages, and currents.

The optical isolator circuit 50 includes an opto-isolator 60, alsoreferred to as an optocoupler, a driver 65, and a current amplifier 70.The opto-isolator comprises an infrared LED that illuminates a pair ofmatched PIN photodiodes in a bifurcated arrangement. One of thephotodiodes provides the output signal, and the other photodiode is usedfor feedback in the driver circuit to servo the LED drive current. Thephotodiodes are reverse biased so that they operate in a photoconductivemode where the current is directly proportional to the incident opticalflux.

The driver 65 includes an operational amplifier U1A having anon-inverting input coupled to a transmit line 22 through acurrent-limiting resistor R1, and having an output coupled through acurrent-limiting resistor R5 to the anode the opto-isolator LED. Theanode of the feedback photodiode is coupled to the inverting input ofthe operational amplifier U1A. This feedback connection causes theoperational amplifier to provide enough LED current that the feedbackphotodiode current follows the voltage at the inverting input to trackthe signal voltage at the non-inverting input. Since the two photodiodesare matched, the current in the output photodiode tracks the current inthe feedback photodiode and is thus proportional to the input voltage onthe transmit line 22.

The current amplifier 70 includes an operational amplifier U2A that isconfigured as a transimpedance amplifier, i.e. a current-to-voltageconverter, and a Darlington transistor pair Q1 that is configured as avoltage-to-current converter. The operational amplifier has anon-inverting input that is held at a fixed DC bias VF. The base of thetransistor is coupled to the operational amplifier output, and thecollector of the transistor is coupled to the L+ line 35. The emitter ofthe transistor is connected through a blocking capacitor C11 and aresistor R22 to the L- line 37 and through a feedback resistor R8 andfilter capacitor C4 to the operational amplifier inverting input, and tothe photodiode anode. The operational amplifier drives the transistor tomaintain the emitter voltage at a level proportional to the photodiodecurrent. Thus, the current through the transistor is a scaled up versionof the photodiode current. The transistor sets up currents in the L+ andL- lines that are proportional to the voltage on the transmit line 22.

The optical isolator circuit 52 includes an opto-isolator 80, a driver85, and a current-to-voltage amplifier 90. The driver 85 includes anoperational amplifier U3A that has a non-inverting input coupled toreceive the output signal from the hybrid 55, and that has an outputcoupled to the anode of the LED in the opto-isolator 80. The anode ofthe feedback photodiode is coupled to the inverting input of theoperational amplifier U3A to provide the same servo control describedabove in connection with the opto-isolator 60.

A current-to-voltage converter 90 comprises an operational amplifier U1Bthat has a non-inverting input coupled to the cathode of the outputphotodiode in the opto-isolator, and that has an output coupled to thereceive line 23. The output impedance of the operational amplifier islow enough that the output signal can be used directly by the userdevice.

Optical isolation circuits provide an acceptable amount of common moderejection for most applications, typically on the order of -50 dB.However, a common mode rejection ratio of at least about 65 dB to 70 dBis required on many telephone lines in the United States. One feature ofthe invention is the provision of a high pass filter having a corner at300 Hz, such that the filter is adapted to reject signals at 60 Hz andharmonics thereof (e.g. power line interference). The high pass filterconsists of operational amplifier U1B, having a filter coupled to itsinput, comprising resistors R28, R33, and R36, and capacitors C14, C16,and C17. It has been found that employing such filter improves thecommon mode rejection ratio by about 25 dB.

In some applications of the invention, it is also desirable to provide ahigh pass filter in the transmit path to improve the common moderejection ratio along this path. In such applications, a high passfilter comprising a circuit similar to that discussed above with regardto the receive path is inserted in the receive signal path between thecapacitor C2 and the resistor R7, as indicated by numeric designator 71.

Optical coupler diodes are typically biased at about 7 mA. When the loopcurrent drops below about 40 mA, for example if the line voltage acrosslines 15T and 15R drops below 7 volts, the current at the opticalcoupler diode falls below 7 mA. It has been found that a loop currentbelow about 30 mA to 40 mA may cause signal distortion in the receivesignal path, especially at higher signal levels, due to low current inthe optical coupler diode. However, many telephone lines cannot supplyenough current to drive both the load and the optical coupler diode. Forexample, it is not uncommon for telephone lines to have loop currents ofabout 20 mA or less.

The invention provides a constant voltage source to the receive opticalcoupler diode 80 (pin 3) in the form of a 3.9 volt reference voltagethat is supplied by a load and current control circuit. The load andcurrent control circuit provides a high AC impedance voltage regulatorthat consists of a current source derived from the loop current andsupplied by the transistor Q2, a Zener diode CR1 having a Zener voltageof 3.9 volts, and resistors R14 and R15. Current is supplied to biastransistor Q2 by the voltage divider consisting of resistors R12, R18,and R19. The Zener diode CR1, resistor R17, and capacitor C5 establish a3.9 volt reference at the node labeled +3.9 V. In this way, a constantreference voltage is supplied to the receive path photo-coupler diodewithout regard to low or varying loop current.

A DC load on the L+ and L- lines is provided by the Darlingtontransistor Q2 and a resistor network. A voltage divider comprising theresistors R12, R18, and R19 establishes the voltage on the base of theDarlington transistor Q2. This voltage, along the resistor network,determines the current flow. For further information, see the discussionof the load circuit herein.

The invention allows the use of an opto-isolator circuit on telephonelines having low or varying loop currents. The load and current controlcircuit provide a simulated inductor that does not block or interferewith the AC component of the loop signal, but that is used to simulate aDC resistance to shape the DC resistance slope. Because the load andcurrent control circuit has a high AC impedance, the circuit does notaffect the AC performance of the telephone line to which it isconnected. Thus, the invention provides a high AC impedance voltageregulator that derives a constant voltage from a variable loop current.

The load and current control circuit is also used in the invention togenerate a reference voltage VF that is supplied to the non-invertinginput of operational amplifier U2A, which is the AC line driver for thetransmit path. By providing a regulated reference voltage to theoperational amplifier, the load and current control circuit stabilizesoperation of the operational amplifier and thereby minimizes distortionin the transmit path.

FIG. 3 is a graph plotting a load resistance window (line voltage vs.line current) for a U.S. telephone standard 330 and a German telephonestandard 320; and FIG. 4 is a graph plotting a load resistance window(line voltage vs. line current) for a United Kingdom telephone standard350. The load and current control circuit is also instrumental inadapting the opto-isolator circuit for operation in the various worldtelephone standards, such as shown in FIGS. 3 and 4.

As can be seen from FIG. 3, the resistance window for the Germanstandard has a steeper curve than that of the United States. The slopeof this curve is a function of the value of the circuit 118 that iscoupled to the opto-isolator circuit at the nodes identified by numericdesignators 120, 122, and 125 via the terminals identified by thenumeric designators 121, 123, and 124. The circuit 118 includes thetransistor Q3 and the resistor R16. A tolerance window is shown in FIG.3, as indicated by the lines identified by the numeric designator 310.Thus, the German resistance window curve must fall within the areadefined by the lines 310. Changing the value of resistor R16 adjusts theload slope to provide compatible curves for other countries.

Likewise, load slopes for different tolerance windows can be provided,such as the load slope for the United Kingdom, that has a tolerancewindow defined by the lines identified by the numeric designator 340, asis shown in FIG. 4. In the case of the U.S. and U.K. slopes, it is notnecessary to include the circuit consisting of the transistor Q3 and theresistor R16 because the slope for these standards is shaped by theresistors R14 and R15, which comprise a parallel network that is soconfigured to allow the use of in the circuit of smaller resistorshaving a lower power rating.

It is therefore an important feature of the invention to improve priorart opto-islator circuits by including a load and current controlcircuit that provides an AC line driver reference to reduce distortionin the circuit transmit path; a regulated voltage source to supplyadequate bias current to the receive path opto-islator diode and therebyreduce receive path distortion in systems having low loop currents; anda variable load slope control that allows the opto-isolator circuit tobe adapted for use in any of the various load slopes and curves usedthroughout the world.

The outgoing signal is removed at the emitter of the transistor Q1.During transmission a differential signal is developed across thetransistor Q1. The resulting voltage at the emitter is communicatedthrough a blocking capacitor C11 and a resistor R22 to the invertinginput of the operational amplifier U2B that provides an output voltagethat is representative of the outgoing signal.

The hybrid 55 provides a voltage signal equal to the superposition ofthe outgoing and incoming currents on the line, and also removes adesired fraction of the outgoing signal. For voice communications, asmall amount of the outgoing signal, e.g. the speaker's voice, isallowed to pass through the receive channel to allow the speaker to hearhis voice. For applications that use modems and the like, the circuiteliminates substantially all of the outgoing signals from the receivechannel. This is dependent on the setting of resistors R21 and R22.

The hybrid circuit differential amplifier derives its signal from avoltage proportional to the total current flowing in the line generatedacross the transistor Q1 that is interposed in the L+ line 35. Thedifferential amplifier includes an operational amplifier U2B and aresistor network R21, R22, R25. The voltages at the emitter andcollector of the transistor Q1 (pins 2 and 3) are communicated throughrespective blocking capacitors C11 and C12 and respective resistors R22and R21 to the inverting input of the operational amplifier U2B thatprovides an output voltage that is proportional to the superposition ofthe outgoing and incoming signals, as seen at the emitter and collectorof the transistor Q1.

In many countries, a complex termination impedance is required forstandard circuitry. The known approach to provide these impedances hasbeen to provide a resistive adjustment to the impedance value. However,many of the world's various telephone systems require both a resistiveand a capacitive component. For example, the United Kingdom sets theminimum return loss using a reference termination standard.

The invention includes a termination circuit (114/116) that is coupledto the opto-isolator circuit of the invention at the nodes identified bynumeric designators 90 and 91. For the U.S. standard, the terminationcircuit consists of the resistor 114 that is coupled to theopto-isolator circuit via terminals 94 and 95; and for the U.K. standardthe termination circuit consists the capacitors C9 and C15, and theresistors R11 and R20 that are coupled to the opto-isolator circuit viaterminals 92 and 93.

A compensation circuit (110/112) is coupled to the opto-isoaltor circuitat the node identified by the numeric designator 96. The compensationcircuit consists of a resistor 110 in the case of the U.S. standard, andthat consists of a capacitor C3, and the resistors R9 and R10 in thecase of the German and UK standards, provides compensation for thetermination circuit, such that the signal path exhibits flat transmitfrequency response. The values of the termination components are afunction of the complex impedance requirements set by the country inwhich the opto-isolator circuit is used. The complex impedancerequirements themselves are a function of the impedance network.

One feature of the invention derives a signal across the transistor Q1,i.e.. a signal is taken from the emitter of transistor Q1 and aninverted signal is taken from the collector of transistor Q1. Thisarrangement reduces trans-hybrid losses while providing a very linearsignal that does not introduce distortion into the cancel equation ofthe input of operational amplifier U2B. The feedback path defined by thecapacitor C4 and the resistor R8 reduces distortion in the operationalamplifier U2B. Poor hybrid return loss in opto-isolator circuits makesmodem digital signal processors ineffective in detecting low levelincoming signals.

Although part types and values are subject to some variation, thoseshown in the schematic represent the presently preferred embodiment. Theopto-isolators are commercially available from the OptoelectronicsDivision of Siemens Components, Inc., Cupertino, Calif. under the partnumber IL300. Opto-isolators are subject to wide manufacturing variancesin their primary to secondary transfer ratio. The invention providestransmit insertion loss and receive gain for the opto-isolators byadjusting the values of the resistors, R4 and R25, at the invertinginputs of the opto-isolator drivers, U1A and U3A, respectively.Representative values are provided below. These values may beestablished by substituting various discrete resistors, or by activelytrimming the resistors to obtain an optimum value. The preferred valuesfor various opto-isolators may be in the range of about 30.9 kΩ to about42.2 kΩ for resistor R4, and from about 60.4 kΩ to about 82.5 kΩ forresistor R25.

The operational amplifiers are standard parts, in dual or quad packages.The transistors are surface mount devices. The resistors can be surfacemount devices, or they can be fabricated by thick film techniques on aceramic substrate if an additional level of miniaturization is required.The unpolarized capacitors (below 1 μf) are ceramic type, while thepolarized capacitors (1 μf and above) are tantalum type.

The line coupler 25 includes an off-hook relay U6 that is interposed inthe ring line 15R, and is controlled by the off-hook signal ascommunicated through a transistor Q4. Ring detect is provided via anopto-isolator U7 whose LED is connected to an input network thatincludes a blocking capacitor C8 and Zener diodes CR5.

In some countries, for example in the United Kingdom, a three wire ringcircuit is required. One feature of the invention provides a three wirering circuit that includes a resistor R29 across which a connection isbrought out to a separate pin P2. This arrangement adapts theopto-isolator circuit to the 3-wire standard. By connecting the pin tothe TIP it matches the standard 2-wire ring circuit.

The ring detect signal is maintained at +5 volts relative to equipmentground and is pulled low when current flows through the opto-isolatorLED. Surge protection is provided by a voltage limiter CR8 across tipand ring lines 15T and 15R. This arrangement may require two additionalresistors or fuses 150, 151 to insure compatibility with internationalsafety standards.

An isolation system must provide internal loss reduction to assureproper connection between the user equipment and the telephone network.A bridge rectifier, consisting of Shottky diodes CR3 and CR4 is coupledbetween the tip and ring lines to ensure proper polarity of the L+ andL- signals, regardless of the polarity with which the line coupler isconnected to the network. The use of Shottky diodes improves looptermination, reduces the internal resistance by about 50 ohms, andthereby allows the opto-islation circuit to be used on longer centraloffice loops.

In conclusion, it can be seen that the invention provides severalsignificant improvements to an otherwise very effective isolationtechnique. Although the invention is described herein with reference tothe preferred embodiment, one skilled in the art will readily appreciatethat other applications may be substituted for those set forth hereinwithout departing from the spirit and scope of the present invention.Accordingly, the invention should only be limited by the claims includedbelow.

We claim:
 1. In an isolation circuit for connecting analog transmit andreceive channels in a user device to a line pair for full duplexcommunications on the line pair while providing an electrical isolationbarrier between the user device and the line pair, comprising:transmitoptical isolation means, having an input located on the user device sideof the barrier and an output on the line side of the barrier and beingresponsive to signals at its input, for generating analog signals at itsoutput linearly corresponding to analog signals at its input; saidtransmit optical isolation means comprising . .a monolithic.!. .Iadd.an.Iaddend.opto-isolator having an LED and first and second photodiodes.., each photodiode, on being reverse biased, delivering a currentproportional to the optical flux incident thereon,.!. and a drivercoupled between the transmit channel and said opto-isolator in afeedback configuration . .such that each of said first and secondphotodiodes delivers a current proportional to a voltage signal from thetransmit channel.!.; receive optical isolation means, having an inputlocated on the line side of the barrier and an output on the user deviceside of the barrier and being responsive to signals at its input, forgenerating analog signals at its output linearly corresponding to analogsignals at . .it.!. .Iadd.its .Iaddend.input; means for coupling theinput of said transmit optical isolation means to said transmit channel;means for coupling the output of said receive optical isolation means tosaid receive channel; and duplexing means coupled to the line pair fordirecting signals at the output of said transmit optical isolation meansonto the line pair, for directing signals on the line pair to the inputof said receive optical isolation means; and for preventing at least aportion of any signals on the line pair that correspond to signalsgenerated at the output of said transmit optical isolation means fromreaching the input of said receive optical isolation means; theimprovement comprising:load and current control means for providing ahigh AC impedance regulated reference voltage derived from loop currentto a receive path optical isolation means photodiode, whereby saidphotodiode receives .Iadd.adequate .Iaddend.bias . .adequate.!. currentto avoid receive path distortion independently of line current level;said load and current control means providing a regulated referencevoltage derived from said loop current to a transmit path line drivercircuit, whereby transmit path distortion is avoided under conditions ofvarying load impedance.
 2. The circuit of claim 1, further comprising:anadjustable load DC impedance circuit for selectively shaping anisolation circuit load DC impedance slope.
 3. The circuit of claim 1,further comprising:high pass filter means associated with said opticalisolation means in at least one of said transmit path and said receivepath, and adapted to provide selected common mode rejection in saidpath.
 4. The circuit of claim 1, further comprising:adjustabletermination means for selectably matching said isolation circuit tocomplex load impedances.
 5. The circuit of claim 1, furthercomprising:means for providing transmit insertion loss and receive gaincompensation for said optical isolation means.
 6. The circuit of claim1, further comprising:means for reducing loop DC termination loss ofsaid isolation circuit.
 7. The circuit of claim 1, furthercomprising:trans-hybrid return loss means for reducing internalisolation circuit losses.
 8. The circuit of claim 1, furthercomprising:pin-out means for adapting said isolation circuit for usewith 3-wire ring circuitry.
 9. The isolation circuit of claim 1,including opto-isolators for providing an electrical barrier between atelephone network and user telephone equipment, said circuit furthercomprising:a load and current control circuit for reducing signaldistortion and insensitivity to low and varying loop current, and lackof complex impedance compensation/termination for diverse telephonenetwork parameters, said load and control circuit comprising a high ACimpedance voltage regulator that derives a constant voltage from avariable loop current, said voltage regulator comprising a Zener diodecontrolled voltage regulator that establishes a reference voltage for areceive path opto-isolator, and that establishes a reference voltage fora transmit path line driver circuit; said load and control circuit alsocomprising an adjustable resistive/capacitive network for selectivelymatching device load impedance to said telephone network.
 10. The deviceof claim 9, further comprising:a line coupler circuit including a bridgeShottky diode rectifier circuit coupled between said device and saidtelephone network for reducing device loop termination losses.
 11. Thedevice of claim 9, further comprising:a high pass filter comprising aresistive/capacitive network in either of said device transmit andreceive paths coupled between a respective opto-isolator output and aline driver input for improving common mode rejection ratios in saidpaths.
 12. The device of claim 9, further comprising:an adjustableresistive/capacitive termination network for selectably matching saiddevice impedance to telephone networks having complex load impedances.13. The device of claim 9, further comprising:a trans-hybrid return losscircuit including a transistor having an emitter and a collector coupledto produce a differential output signal that is proportional to thesuperposition of signals along said receive path and transmit paths. 14.The isolation circuit of claim 1, further comprising:a high AC impedanceload and current control circuit for reducing signal distortion andinsensitivity to low signal levels resulting from low circuit current,said load and control circuit including a Zener diode controlled voltageregulator that derives a reference voltage for said opto-isolator from avarying loop current.